2 Observations

The target stars of the time variability program have been selected on the criteria of the infrared brightness and the visibility of ISO. Z Cyg is a Mira variable star of M5e-M9e with the variability period of 264 days (Kholopov et al. 1985-1988). The IRAS LRS spectrum of Z Cyg clearly shows the 10 and 18 $\mu$m silicate features (Joint IRAS Science Working Group 1986). Previous studies of decomposition of the spectra into two components suggest that the mid-infrared spectrum of Z Cyg is located in the extreme end of the variety and it represents a spectrum composed of only the silicate component with no or a very small amount of the aluminum oxide component (Onaka et al. 1989b; Miyata et al. 2000). Z Cyg is also known to have a very large negative LSR velocity (-147.7 km s^-1) and a small terminal velocity (4.0 km s^-1) in the CO radio emission (Young 1995). The mass-loss rate estimated from the CO observation is $4 \times 10^{-8}~M_{\odot}$ yr^-1(Young 1995), while the optical depth at 10 $\mu$m derived from the dust shell model is suggested to be about 0.01 (Onaka et al. 1989b), indicating that Z Cyg is surrounded by an optically thin circumstellar shell.

Z Cyg was located in a good visibility zone of ISO and was observed 7 times with an interval of the variability phase of approximately 0.2 except for the last observation, which had about 0.3 phase interval from the previous observation. The observations of Z Cyg started near a minimum and covered the two following maxima. The observation dates and variability phases $\phi$ are listed in Table 1, where $\phi= 0$ and 1 correspond to maximum and the phase is counted beyond unity to indicate the consecutive nature of the observations. The visual light curve of Z Cyg is slightly asymmetric and has a minimum around $\phi = 0.6$. The observations were made in the SWS01 mode (full grating scan for 2.38-45.2 $\mu$m) with the speed of 2 except for the first observation, in which the speed was set as 1. These observations provided a spectral resolution of approximately 300-500. The spectrum taken at the last observation had missing parts in the range 2.6-3.0 $\mu$m (band 1b) and 12.6-15.0 $\mu$m (band 3a) due to telemetry trouble. The missing parts are small and do not affect the present analysis. Together with the SWS observations broad-band photometry was carried out by PHT in the 3.6, 11.5, and 25 $\mu$m bands (Lemke et al. 1996). At the first observation, far-infrared photometry with LWS (LWS02) was also attempted (Clegg et al. 1996), but the data did not have a sufficient signal-to-noise ratio. In the later observations the LWS photometry was not executed.

   

Table 1: Observation date and phase of Z Cyg.

Observation date	Variability phase $\phi^1$
1996 August 5	0.55
1996 October 8	0.79
1996 November 24	0.97
1997 January 24	1.20
1997 March 21	1.42
1997 May 15	1.63
1997 August 8	1.93

¹ The variability phases are estimated from the AAVSO data.

The Off-Line Processing (OLP) version 10.1 data were used for the SWS observations. The SWS spectra were reduced by the Observers SWS Interactive Analysis Package (OSIA) version 2.0. The data of the PHT observations were reduced with the PHT Interactive Analysis (PIA) software package version 9.1, in which both the calibration based on the internal fine calibration sources (FCSs) and the default calibration (Laureijs et al. 2001) were applied to estimate the internal accuracy. The final PHT data of Z Cyg were obtained by subtracting the background flux taken at 5 arcmin away from Z Cyg in the concatenated observations. Some of the SWS spectra show apparent gaps between the band boundaries. The band 3a often appears faint compared to the contiguous bands. We corrected the gaps by scaling the band flux referring to the band 1 flux, which has the least uncertainty in the flux calibration (Leech et al. 2001). Comparison with the PHT data confirms that the corrections do not introduce spurious effects (see Appendix A).

Figure 1: Observed spectra of Z Cyg at 7 different epochs. The variability phase estimated from the visual light curve is indicated in each figure (Table 1). All the figures have the same scale. Open diamonds are the PHT data based on the default calibration, while open circles are those based on the FCSs. The thin smooth lines indicate the best fit model with y0 =0.1 and the dust emissivity Q7 (see Sect. 3). The dust emissivity is derived from the spectrum at $\phi = 0.97$ and thus the model fits perfectly with the observed spectrum.

Figure 1 shows the SWS spectra of Z Cyg taken at 7 different phases. Large variations in the infrared region are clearly seen with the visual variability. In this paper we concentrate on the dust emission for $\lambda > 7$ $\mu$m. The spectra of the range 2.4-7 $\mu$m also show significant variations, which provide information on the physical properties of the photosphere as well as the outer atmosphere. Investigations on the short wavelength range are reported in Matsuura et al. (2002). The mid-infrared spectra of Z Cyg show a clear trend that the circumstellar emission in the 10-20  $\mu$m region increased relative to the photospheric emission of around 4 $\mu$m from minimum to maximum. The ratio of the 10 $\mu$m to 18 $\mu$m bands also increased, indicating an increase in the dust temperature at maximum. The band ratio decreased as the star went to the second minimum (the variability phase $\phi = 1.63$), where the spectrum became quite similar to the spectrum at the first minimum ( $\phi = 0.79$). Then at the second maximum ( $\phi = 1.93$) the star showed a spectrum nearly the same as that at the first maximum ( $\phi = 0.97$). The variation in the infrared spectrum of Z Cyg appears to synchronize with the visual light variation.

The PHT data are plotted together in Fig. 1, taking account of the color corrections. We found that the SWS flux agrees with the PHT flux of the default calibration within 10% in most cases. The accuracy of the absolute flux calibration of SWS is estimated to be in a similar range (12-15% for $\lambda > 4$ $\mu$m at present, Leech et al. 2001). The general trends described above are also indicated by the PHT data, confirming the variations seen in the SWS spectra.